The present invention relates generally to the field of fluid drop generation and the application thereof to jet drop recorders of the type shown in U.S. Pat. No. 3,701,998 to Mathis, issued Oct. 31, 1972. In recorders of this type, one or more rows of orifices in a plate receive an electrically conductive recording fluid, such as a water base ink, from a pressurized fluid manifold and eject the fluid in parallel streams. The streams are broken up into drops as a result of the application of a series of travelling waves to the plate or as a result of other mechanical stimulation, such as the application of compression waves to the fluid in the manifold. Graphic reproduction in recorders of this type is accomplished by selectively charging and deflecting some of the drops in each of the streams and, thereafter, depositing drops on a moving print receiving medium, such as a moving web or moving sheets of paper.
Charging of the drops is accomplished by the application of charge control signals to charging electrodes near the edge of the drop streams. Charges are induced in the ends of fluid filaments emerging from the orifices by the charge control signals. As the drops separate from their parent fluid filaments, they carry a portion of the charge applied by the charging electrodes. Thereafter, the drops pass through one or more electrostatic fields which have no effect on the uncharged drops but which cause the charged drops to be deflected in proportion to the level of charge which they carry. Drops which are not to be printed are charged sufficiently such that they are deflected to a catcher which services all of the drop streams.
One problem encountered in the field of ink jet printers has been attaining sufficient image resolution. Since a discrete number of drops are applied to a print medium to form the images, it is clear that image definition may be improved by increasing the number of drops and providing a proportionate increase in data handling capability. If, however, only one print position on the print medium is serviced by each orifice, the number of print positions per unit width and, therefore, the resolution of the image in the direction transverse to the direction of medium movement is limited by the minimum dimensions required between adjacent orifices. The approach taken in the Mathis device is to provide two rows of drop streams, with each row of streams being perpendicular to the direction of medium movement, and the drops from one row of the drop streams servicing print positions which interlace with those serviced by the other row of drop streams. The charging of drops in the two rows is timed such that printing from the two rows of streams is in registration. The separation between adjacent streams in each of the rows is therefore twice that which would be required in a one row printer of comparable resolution.
U.S. Pat. No. 4,085,409, issued Apr. 18, 1978, to Paranjpe, discloses a printer which is somewhat similar in construction to that of the Mathis printer. The rows of jets in the Paranjpe '409 printer are positioned along parallel lines which are inclined to the direction of web movement. Drops in each of the jet drop streams are selectively charged to any of several charge levels such that they are deflected to service a number of print positions. The inclined printer of Paranjpe '409 provides improved resolution across the width of the web, both as a result of positioning the rows of jets along lines which are inclined with respect to the movement of the print web and by virtue of the fact that each jet services a number of print positions. Because of the deflection electrode position in the Paranjpe printer, the deflection fields are created by electrodes which do not extend between adjacent jets. The inter-jet spacing is, therefore, not limited by the deflection electrode structure.
In a printer, such as shown in the Paranjpe '409 patent, the drops from the drop streams may be accurately deposited at print positions on the medium and thus provide a high resolution print image across the width of the medium, providing that each of the jets is accurately positioned along the line of jets. Should one of the jets in such a system be slightly crooked, the band of print lines which it services will be laterally displaced from the desired position with the result that a small gap between the band serviced by the crooked jet and one of the adjacent bands of print lines will be produced in which no printing may be accomplished. Since these unprinted areas will appear as white streaks running parallel to the direction of movement of the print medium, even small interband gaps will be noticeable and will detract significantly from the appearance of the final print image. Since all of the print lines serviced by a jet are shifted to the same degree when the jet becomes crooked, however, a misaligned jet does not produce white streaks between the print lines serviced by the jet.
U.S. Pat. No. 4,060,804, issued Nov. 29, 1977, to Yamada, discloses an ink jet printing device in which two jets are provided for printing at two adjacent groups of print positions. In order to minimize the possibility of a gap or other deterioration along the boundary of the bands of print lines serviced by the two jets, the jets are positioned such that the print lines adjacent the boundary in each of the groups are serviced by the drops from the respective jets which are deflected the least. This arrangement is said to minimize the likelihood of a white streak occurring as a result of errors in the deflection of the drops. If one or both of the jets are crooked, however, this scheme will not provide an improvement in image quality.
U.S. Pat. No. 4,219,822, issued Aug. 26, 1980, to Paranjpe, shows an ink jet printer in which drops of ink from a plurality of jet drop streams, arranged in a row, are selectively deposited on a moving print receiving medium. The row of streams is inclined with respect to the direction of movement of the medium. The drops from each stream are deflected in a direction perpendicular to the row of streams to a plurality of print positions. Drops deposited at each print position define a corresponding print line along the print receiving medium. At least two of the print lines serviced by each jet drop stream overlap substantially with print lines serviced by adjacent jet drop streams. Thus, each jet drop stream services a band of print lines with at least one print line on each edge of the band overlapping with those of adjacent bands. This arrangement makes the occurrence of white streaks due to crooked jets much less likely. In order to prevent distortion of the image, however, the same image data is used to control printing along the sets of overlapping print lines.
It will be appreciated that the Paranjpe '822 device is limited in that two print lines, serviced by each jet, print image information which is redundantly printed by adjacent jets as well. Further, Paranjpe '822 teaches providing the same number of drops per inch in both vertical and horizontal directions which constrains the angle of the row of jets unduly.
It is seen, therefore, that a need exists for a printer in which each jet services a number of print positions forming a band of print lines along the print web and in which the effects of a crooked jet on the resulting print image are minimized.